Plant for exploiting geothermal energy

ABSTRACT

A plant for exploiting geothermal energy by circulating water through a geological formation ( 1 ) at a substantial depth below the earth surface ( 2 ), comprises at least one supply hole ( 3 ) leading from the surface ( 2 ) down to said formation ( 1 ) and at least one return hole ( 4 ) for transporting heated water from the formation to the surface. The supply and return holes ( 3, 4 ) are interconnected by a plurality of heat absorbing holes ( 5 ) which are spaced more than 50 m apart. The heat absorbing holes ( 5 ) have a total length of many kilometers but a relatively small diameter in the order of 10 cm. A method for designing the plant involving determining the dimensions of the heat transfer holes ( 5 ) is also disclosed.

The present invention relates to a plant for exploiting geothermalenergy by circulating water through a geological formation at least 1000m below the earth surface, comprising at least one supply hole leadingfrom the surface down to said formation, at least one return hole forthe transport of heated water from said formation to the surface, and aheat absorbing arrangement connecting the supply and return holes, saidarrangement comprising a heat transfer surface across which heat istransferred from said formation to said water.

An example of such a plant is disclosed in U.S. Pat. Nos. 3,863,709 and4,223,729, the latter and other patents mentioned therein being includedby reference. U.S. Pat. No. 4,223,729 relates to the exploitation ofgeothermal energy from hot dry rock (HDR) formations. Due to the lowthermal conductivities of such formations, it has been the generalbelief that thermal energy could not be extracted at a useful ratewithout a very large heat transfer surface being available in thegeological formation.

Up to now, in almost all known plants in HDR one has tried to obtainsuch very large heat transfer surface areas by creating fracture zonesbetween the supply and return holes, either by expanding existingfracture zones, by blasting the rock between the holes using explosives,or by establishing a fracture system through the use of hydrostaticpressure and/or heat. Even if such fracture zones could be established,they provide rather unpredictable flow conditions for the water sincethe water tends to take the path of least resistance and therefore notflow through the narrower fissures of the fracture zone.

Quite contrary to the common belief, the present inventors havesurprisingly realized that if a geothermal plant of the type in questionis to have any useful life, the magnitude of the heat transfer surfacearea is not a critical factor. Instead, the decisive factor is theavailability of a large volume of rock in close proximity to the heattransfer surface. Thus, the inventors believe that a geothermal plant,e.g. designed for heating and hot water purposes, should have at least15,000 m³ of rock located within 50 m of the heat transfer surface forevery kW the plant is to deliver. For smaller plants in unfavourablerock conditions, this volume may be more like 60,000 m³/kW.

Consequently, in one aspect of the invention, it provides a plant forexploiting geothermal energy of the kind defined in the introductoryparagraph above, the plant being characterized in that it has a givennominal power in MW defined as the heat to be absorbed by saidarrangement after one year of operation, in that said heat transfersurface comprises at least one drilled heat absorbing hole, and in thatthe volume of said formation located within 50 m of said heat absorbinghole, is at least about 10×10⁶ m³, preferably at least 20×10⁶ m³,multiplied by said nominal power.

These numbers represent a much larger mass of rock than contemplated byany prior art plant with an economically viable output.

The inventors have found that the most efficient way of establishing asufficiently large volume of rock in close proximity to the heattransfer surface would be to use a drilled hole for said surface.However, such a hole would have to be quite long to penetrate therequired volume of rock, and the drilling costs would appear prohibitivesince one still had to assume that a substantial heat transfer surface,i.e. large diameter hole, would be necessary to provide the requiredheat flux from the rock to the water circulating through the hole.

Nevertheless, the inventors set out to calculate the heat transfer froma large cylinder of rock into water flowing in a central hole of thecylinder using the differential equation presented by H. S. Carslaw andJ. C. Jaeger in “Conduction of Heat in Solids”, Second Edition, Oxford,which is hereby included by reference, a task that to their knowledgenobody had done before. Surprisingly, they found that in the course of30-40 years, the temperature in the rock at a distance of 100 m from thehole would hardly change at all. Even more surprisingly, they found thatthe available energy could be extracted over this time period with ahole which, from a heat transfer point of view, had a diameter as smallas 10 cm and even less. Further analysis showed that increasing the holediameter to 1 m, which would increase the heat transfer surface tenfold,would less than double the possible heat extraction rate, otherconditions being held equal. However, the cross-sectional area of such ahole, and therefore the mass having to be removed to make the hole,would increase 100 times. Consequently, the most economical solutionseemed to be to use the smallest hole diameter that could be drilledover long distances, which with the current technology is limited toabout 10 cm.

However, the length of such a heat absorbing hole would normally exceed5 km even for the smallest practical economical plant and, in addition,the supply and return holes could extend for much the same distance.Besides, the pressure drop and consequent pump losses could be too largefor very long slender holes. To solve this problem, the inventors havesuggested to divide the heat absorbing hole into a plurality of passesconnected in parallel and being spaced sufficiently apart to haveavailable a sufficient volume of rock to supply the desired heat throughthe required lifetime of the plant.

Thus, according to a second aspect, the invention provides a plant forexploiting geothermal energy by circulating water through a geologicalformation at least 1000 m below the earth surface, comprising at leastone supply hole leading from the surface down to said formation, atleast one return hole for the transport of heated water from saidformation to the surface, and a heat absorbing arrangement connectingthe supply and return holes, the plant being characterized in that saidheat absorbing arrangement comprises a plurality of drilled heatabsorbing holes connecting the supply and return holes in a parallelflow manner, a substantial part of each heat absorbing hole lying atleast 50 m, preferably at least 100 m, from the nearest heat absorbinghole, the total length of the heat absorbing holes preferably exceeding5000 m.

According to a further aspect of the present invention, a plant forexploiting geothermal energy of the type recited in the classifying partof the above paragraph is characterized in that said heat absorbingarrangement comprises a plurality of drilled heat absorbing holesarranged in parallel flow relationship, said heat absorbing holesextending at an angle downwards from the supply hole to the return hole.

The sloping of the heat absorbing holes makes them simpler to drill, forinstance by using a water driven percussion hammer and coiled tubing.The weight on the drill bit can more easily be controlled since thefriction of the coiled tubing against the hole wall can substantiallysupport the weight of the tubing. This may considerably increase drillbit life and reduce drilling costs.

Since the rock temperature increases with increasing depth, letting thewater flow direction be downward through the sloping holes will permitthe temperature increase in the water to follow the temperature increasein the surrounding rock, thus keeping the temperature difference betweenthe rock and water substantially constant. This may be likened to acountercurrent flow heat transfer arrangement and will allow for ashorter length of heat absorbing hole and an optimum absorption of heatfrom the rock. A countercurrent flow arrangement would be expected toproduce heat from the same rock volume for 2-3 times longer than anequivalent co-current heat transfer arrangement.

The magnitude of the sloping angle will depend on several factors, forinstance the temperature gradient in the rock, the length of the heatabsorbing hole and the water flow rate. Calculating the angle will bewell within the capabilities of the skilled person and will thereforenot be detailed here. The angle would normally lie between 20° and 50°,preferably it will be about 40°.

Furthermore, in order to maximize the extraction of heat from a givenvolume of rock, at least substantial parts of the heat absorbing holesextend parallel to each other. More preferably, the heat absorbing holesare arranged in one layer or, if necessary, in a plurality of verticallyspaced layers. Providing an array of vertically spaced layers, eachlayer having a plurality of heat absorbing holes, allows for increasingthe capacity of the plant without spreading the holes over a wide area.This is of considerable importance if the volume of earth available forexploitation is not large.

Preferably the distance between adjacent holes in each layer is the sameas the distance between adjacent vertically spaced holes, this distancebeing at least 50 m, preferably at least 100 m. On the other hand, thespacing should be less than about 150 m to limit the physical extent ofthe plant. A plant in accordance with the invention may have just asingle supply hole and a single return hole. However, the plant may havea plurality of holes arranged, most preferably circumferentiallyequispaced, around a common return hole. In one particular embodimentfor example three supply holes may be arranged around a single returnhole. It should be noted that the return hole may be a single drilledhole or a cluster of closely spaced, smaller diameter holes, whichexhibit substantially the same heat loss characteristics as a largerdiameter single hole.

Preferably the upper ends of the supply hole and the return hole may bearranged close to one another, with the holes diverting downwards so asto introduce a substantial spacing between the ends of the heatabsorbing holes extending therebetween. Preferably this spacing is atleast 500 m. Such a plant arrangement allows for a compact plantconstruction at the surface but at the same time permitting thenecessary heat absorbing hole length to be obtained. Since the heatabsorbing holes normally will be drilled starting from the supply hole,the supply hole may advantageously be made generally vertical, thuspermitting the heat absorbing holes to reach the greatest possible depth(and temperature) permitted by a given maximum length of the drillstring available for drilling the holes. Using a coiled tubing, thepractical maximum length may be 6-8000 m.

As mentioned earlier, it has previous been considered necessary to havevery large heat transfer surfaces to extract thermal energy, asexemplified by the large fracture surface attempted according to U.S.Pat. No. 4,223,729. However, since the inventors very unexpectedly havefound that heat can be extracted quite satisfactorily through heatabsorbing holes having a diameter in the order of 10 cm or even less,this is in itself believed to be a novel arrangement, so that inaccordance with a yet further aspect of the present invention, there isprovided a plant of the type disclosed in the introductory paragraph,the plant being characterized by a heat absorbing arrangement comprisinga plurality of drilled heat absorbing holes having a diameter of lessthan 14 cm, and preferably a nominal diameter of 10 cm.

The invention also provides a method for use in designing a geothermalplant, the method being defined in claim 15.

Further advantageous features of the invention are defined in thedependent claims.

It has come to the attention of the inventors that WO 96/23181 disclosesan attempt to utilize abandoned offshore oil wells in extracting thermalenergy, which in turn is supposed to be converted to electric power andsupplied to a consumer. Here, two 3000 m deep wells are used for thesupply and return holes, respectively, the wells being interconnected attheir lower ends by a generally horizontally drilled loop which is 1000m long and has a diameter of 21.5 cm. 700 m³/h of water are circulatedthrough the loop with an inlet temperature of 20° C. The publicationsimply assumes that the water will return at a temperature of 90° C.,which is the temperature of the formation where the connecting loop issituated, and thus provide 40 MW of thermal power. This assumption isgrossly inaccurate. Using their method referred to above, the inventorshave found that the return water temperature would be just a few degreesabove the supply temperature and that the loop would have to be morethan 60 times longer in order to provide 40 MW. This clearly goes toshow the usefulness, importance and surprising effect of the presentinvention.

For better understanding of the invention it will be described withreference to the exemplifying embodiments shown in the appendeddrawings, wherein:

FIG. 1 is a schematic perspective view of a geothermal plant accordingto the invention,

FIG. 2 is a schematic side view of the plant of FIG. 1,

FIG. 3 is a schematic plan view of a larger geothermal plant accordingto the invention, and

FIG. 4 is a section along the line IV—IV in FIG. 3.

In the two embodiments shown in the drawings, the same referencenumerals have been used for similar parts.

The plant illustrated in FIGS. 1 and 2 is for the major parts located ina geological formation 1 below the earth surface 2. The formation has aheat conductivity of 3 W/m° C. The plant comprises a supply hole 3 witha diameter of 15 cm and a return hole 4 with a diameter of 15 cm. Thesupply and return holes 3, 4 are interconnected by four heat absorbingholes 5, each having a diameter of 10 cm and being approximately 2000 mlong. The spacing between these holes 5 may be 100-150 m. They have beendrilled starting out from the supply hole 3 and terminate at or near thereturn hole 4. A fracture zone 6 has been established in this area toprovide flow communication between the holes 4 and 5 since it would bedifficult to hit the return hole 4 directly when drilling the heatabsorbing holes 5.

The upper parts of the supply and return holes 3, 4 are provided with acasing 7 extending about 300 m into the ground to seal the holes againstthe surrounding formation in this area.

On the surface, the supply and return holes 3, 4 are connected to oneside of a separating heat exchanger 8, and a circulation pump 9 isprovided in the supply line 3. An auxiliary pump 10 is located at thelower end of the casing 7 of the return hole 4, the purpose of which isto reduce the pressure in the return hole should water tend to leak fromthe holes out into the formation 1.

The other side of the separating heat exchanger 8 is in flowcommunication with various heat consuming appliances exemplified by aradiator 11, a warm air heater 12 and a hot water tank 13.

For reasons of expediency, FIGS. 1 and 2 provide the varioustemperatures, flow velocities and flow rate and dimensions of thegeothermal plant. Furthermore, FIG. 1 indicates that the heat absorbingholes 5 absorb 0.21 kW/m. Considering that the total length of the heatabsorbing holes is about 8000 m, they extract close to 1.7 MW from therock. The same number can be had by multiplying the water flow rate andtemperature difference between the supply and return holes. However, fora plant as small as this, the heat loss from the return hole 4 to thesurroundings will not be negligible so that the net power of the plantwould expectedly be about 1.5 MW.

FIG. 2 shows that the supply hole 3 is vertical and nearly 3200 m long.The heat absorbing holes 5 extend downwards at an angle α equal to about45° to the horizontal. Considering that they are 2000 m long and addingsome length for the curved end portions, the heat absorbing holes couldbe drilled using a percussion hammer and coiled tubing about 5000 mlong.

In determining the number 0.21 kW/m for the heat absorption holes, theinventors have used the result of their calculations mentioned in theintroductory part of this specification. These calculations, including afew approximations of little practical importance, have yielded thefollowing equation:

Q≡K·k ^(0.93) ·D ^(0.2)(0.12+(t+1)^(−0.1))(T _(G) −T _(W))·l  (1)

where:

Q is the heat absorbed from the hole in W

K is a constant between 1.7 and 2.0

k is the heat conductivity of the rock in W/m ° C., typically about 3for dense rock

T_(G) is the initial average temperature of the geological formationalong the heat absorbing hole

T_(W) is the average water temperature flowing through the heatabsorbing hole

D is the diameter of the hole in meters

l is the length of the hole in meters

t is the plant operating time in years

Equation (1) assumes a countercurrent type heat exchange and would notbe accurate in a co-current situation. The small exponent 0.2 for thediameter D indicates the low influence of the diameter on the heatabsorption, while the exponent −0.1 related to the operating timeresults in the plant losing about 1% of power per year after one year ofoperation.

The equation can also be used to calculate the heat loss from the returnhole 4 with a fair degree of accuracy.

Since equation (1) i.a. shows that the effect of the diameter is quitesmall and that for economical reasons, the diameter will be held quitelow, and since the depletion of the heat in the rock available willproceed quite slowly, equation (1) can be further simplified to thefollowing form:

Q≡C·(T _(G) −T _(W))·k·l  (2)

where:

C is a constant between 0.6 and 2.4, the low side being for smallerplants and low temperature gradients in the rock and the high side forlarger plants and high temperature gradients.

The embodiment in FIGS. 3 and 4 is a geothermal plant designed for anominal power of 50 MW and 40 MW average power over 240000 hours of use(60 years at 4000 hours per year). The supply water temperature is 40°C. and the return temperature 100° C., with a water flow rate of about200 kg/sec. The geological formation 1 consists of granite having a heatconductivity of 4 W/m° C. and,the temperature gradient is 30° C./km.

In order to obtain the necessary total length of the heat absorbingholes 5, they have been arranged in three sets 120° apart and feedinginto a common return hole 4. Each set of heat absorbing holes 5consisting of seven layers 14 of heat absorbing holes 5, the layerscontaining three essentially parallel heat absorbing holes. The spacingbetween the holes 5 is about 100 m in the horizontal direction and100-150 m in the vertical direction. Each hole has a diameter of 10 cmand is about 2300 m long. It forms an angle α with the horizontal ofabout 40°. The supply hole 3 feeding each set of heat absorbing holeshas a diameter of 25 cm, and the diameter of the return hole 4 is about40 cm. This means that the water velocity in the return hole 4 is fourtimes the velocity in the heat absorbing holes 5.

For simplicity, the equipment on the earth surface 2 for utilizing theheat produced by the plant has not been shown. If, after many years ofoperation the heat in the rock around the heat absorbing holes 5 hasbeen depleted, new heat absorbing holes can be drilled in the sectorsbetween the existing sets, or they may be drilled below the existingholes. Drilling new holes below the existing ones will also be thenatural way of increasing or renewing the capacity of the plant shown inFIGS. 1 and 2.

It will be noted that the heat absorbing holes 5 in the FIGS. 3 and 4embodiment has a total length of bout 145 km. Yet it has about the samethermal power as expected from one km heat absorbing hole in WO96/23181.

It will be understood that the invention is not limited in any way bythe exemplifying embodiments described above, but may be varied andmodified in a number of ways without departing from the spirit of theinvention and scope of the appended claims.

What is claimed is:
 1. A plant for exploiting geothermal energy bycirculating water through a geological formation at least 1000 m belowthe earth surface, comprising at least one supply hole leading from thesurface down to said formation, at least one return hole for thetransport of heated water from said formation to the surface, and a heatabsorbing arrangement connecting the supply and return holes, saidarrangement comprising a heat transfer surface across which heat istransferred from said formation to said water, said heat transfersurface comprising at least one drilled heat absorbing holecharacterized in that the plant has a given nominal power in MW definedas the heat to be absorbed by said arrangement after one year ofoperation, and in that the length of said heat absorbing hole is atleast about 2600 m multiplied by said nominal power.
 2. A plantaccording to claim 1 characterized in that said heat absorbingarrangement comprises a plurality of drilled heat absorbing holesconnecting the supply and return holes in a parallel flow relationship,a substantial part of each heat absorbing hole lying at least 50 m fromthe nearest heat absorbing hole, the total length of the heat absorbingholes exceeding 5000 m.
 3. A plant according to claim 2 characterized inthat said heat absorbing holes extend at an angle (α) downwards from thesupply hole to the return hole.
 4. A plant according to claim 3,characterized in that said angle (α) is between about 20° and 50°.
 5. Aplant according to claim 2, characterized in that at least a substantialpart of the heat absorbing holes extend parallel to each other.
 6. Aplant according to claim 2, characterized in that the heat absorbingholes (5) are arranged in vertically spaced layers.
 7. A plant accordingto claim 6, characterized in that the distance between adjacent holes ineach layer is about the same as the distance between adjacent verticalholes.
 8. A plant according to claim 6, characterized in that the saiddistance between said layers is less than about 150 m.
 9. A plantaccording to claim 2, characterized in that a plurality of supply holesare arranged around a common return hole.
 10. A plant according to claim2, characterized in that the upper ends of said supply hole and saidreturn hole are arranged closely to one another, one of said holesextending generally vertically and the other deviating from thevertical, so as to introduce a substantial spacing between the ends ofthe heat absorbing holes.
 11. A plant according to claim 10 wherein thesaid spacing is at least 1 km.
 12. A plant according to claim 2,characterized in that the heat absorbing holes have a diameter of lessthan 18 cm.
 13. A plant according to claim 12, characterized in that thediameter of said heat absorbing holes is less than 14 cm.
 14. A plantaccording to claim 12 wherein the holes are of a nominal 4″ (10 cm)diameter.
 15. A method for use in constructing a plant for exploitinggeothermal energy by circulating water through a geological formation atleast 1000 m below the earth surface, said plant comprising at least onesupply hole leading from the surface down to said formation, at leastone return hole for the transport of heated water from said formation tothe surface, and at least one heat absorbing hole connecting the supplyand return holes characterized by dimensioning said at least oneabsorbing hole in accordance with a solution of the differentialequation for heat transfer from a cylinder of homogenous rock materialinto water flowing through a central hole in said cylinder and using theformula Q≡K·k ^(0.93) ·D ^(0.2)(0.12+(t+1)^(−0.1))(T _(G) −T _(W))·l asan approximation to said solution, the portions of the formula havingbeen defined elsewhere in the specification.
 16. A method according toclaim 15, characterized by using the formula Q≡C·(T _(G) −T _(W))·k·l asan approximation to said solution, the factors of the formula (2) havingbeen defined elsewhere in this specification.
 17. A method according toclaim 15, characterized in that at least the major parts of the heatabsorbing holes are drilled by using a percussion hammer and a coiledtubing drill string.
 18. A method for use in constructing a plant forexploiting geothermal energy by circulating water through a geologicalformation at least 1000 m below the earth surface, said plant comprisingat least one supply hole leading from the surface down to saidformation, at least one return hole for the transport of heated waterfrom said formation to the surface, and at least one heat absorbing holeconnecting the supply and return holes, characterized by forming saidheat absorbing hole arranged in a parallel flow relationship, said heatabsorbing holes being dimensioned in accordance with the formula Q≡K·k^(0.93) ·D ^(0.2)(0.12+(t+1)^(−0.1))(T _(G) −T _(W))·l the factors ofthe formula (1) having been defined elsewhere in this specification. 19.A method for use in constructing a plant for exploiting geothermalenergy by circulating water through a geological formation at least 1000m below the earth surface, said plant comprising at least one supplyhole leading from the surface down to said formation, at least onereturn hole for the transport of heated water from said formation to thesurface, and at least one heat absorbing hole connecting the supply andreturn holes, characterized by forming said heat absorbing hole as aplurality of heat absorbing holes arranged in a parallel flowrelationship, said heat absorbing holes being dimensioned in accordancewith the formula Q≡C·(T _(G) −T _(W))·k·l the factors of the formula (2)having been defined elsewhere in this specification.
 20. A plant forexploiting geothermal energy by circulating water through a geologicalformation at least 1000 m below the earth surface, comprising at leastone supply hole leading from the surface down to said formation, atleast one return hole for the transport of heated water from saidformation to the surface and a heat absorbing arrangement connecting thesupply and return holes, said arrangement comprising a heat transfersurface across which heat is transferred from said formation to saidwater, characterized in that the plant has a given nominal power in MWdefined as the heat to be absorbed by said arrangement after one year ofoperation, and in that said heat transfer surface comprises at least onedrilled heat absorbing hole, the total length of which is at least about1300 m multiplied by said nominal power.
 21. A plant according to claim1, characterized in that a circulation pump is connected to the supplyhole and an auxiliary pump is arranged in the return hole.